1. Field of the Invention
This invention relates to a statistical method for estimating the relative separation between a plurality of seismic transducers disposed along a line of survey, using acoustic ranging.
2. Discussion of the Prior Art
In the art of seismic exploration, a plurality of acoustic sensors are emplaced as along a line of survey. The sensors are connected, at prescribed intervals, to a cable that may be as much as three kilometers or more long. At sea, the cable and its sensors are towed behind a ship; on land, the cable is picked up and moved by truck or other vehicle. At designated stations, also known as shot points, an energy source insonifies the surrounding medium (fires a shot) to generate a wavefield in the subsurface. The wavefield is reflected from subterranean earth layers, to return back to the surface as a seismic-signal wavetrain. The returning seismic signals are detected by the sensors which convert the seismic energy to electrical signals. The electrical signals are transmitted, through suitable conductors in the cable, to a signal utilization device for archival recordation. The received electrical signals are processed by well known means to provide a picture of the attitude of the subsurface earth layers.
At sea, various surveying procedures are used. In deep water, the sensors are mounted in a neutrally-buoyant streamer cable that is towed through the water by a ship with the head end of the streamer trailing one or two hundred meters astern of the ship. An acoustic source, towed between the stern and the head end of the cable, fires a shot in the water at selected time intervals that are timed to occur when the ship passes over designated shot points along the line of survey. Both the sensors and the source continuously move together. The location of the towing ship is known accurately from information furnished by radio-location and/or satellite navigation facilities. Self-contained, sophisticated instrumentation in the streamer cable itself continuously tracks the configuration of the cable relative to the position of the energy source.
Two different methods are used in shallow water. In the more conventional method, a bottom cable, also known as a bay cable, is laid directly on the water bottom by a cable boat along a designated line of survey. The sensors may be distributed in linear or areal arrays. Independently of the cable boat, a shooting boat travels along a line parallel to and offset from the bottom cable. At specified shot points or source positions, determined by radio-ranging or satellite locationing, a shot is fired. The positions of the shot points are known accurately but the locations of the sensors are known only approximately.
A more exotic shallow-water method involves sono-buoys. A buoy boat sows a plurality of sono-buoys, to which one or more sensors is connected, in an areal grid pattern. The shooting boat winds its way through the grid, firing a shot at selected shot points. Seismic signals received by the sensors are sent back to a recording ship by VHF transmissions from the buoys. Although the locations of the sono-buoys were known when they were laid out, by the time the shooting boat arrives on the scene, wind, waves and currents will have forced the buoys to drift out of position. Their locations are only approximately known.
Successful processing of the received seismic signals requires accurate knowledge of not only the locations of the sensors with respect to the energy source but also the locations of the sensors with respect to the world as a whole. In deepwater operations both the source and the sensors move together, towed by the same ship. As pointed out above, sophisticated instrumentation monitors the exact configuration of the sensors with respect to the energy source. In shallow water, where the sensors and the energy source are deployed independently, the separation between the energy source and a sensor is, at best, only a guesstimate.
Various methods have been used in shallow-water operations for defining the relative separation between a known source position and the approximately-known sensor positions. For example, in U.S. Pat. No. 4,446,528, in a marine exploration system, a ship measures the water depth to a seismic cable as it passes over the cable. The ship interrogates the sensors in the cable by means of sonar pulses along a slant range as the ship travels along a parallel and horizontally offset path relative to the cable. The locations of the sensors are measured from recordings of the measured water depths and slant-range travel times.
In another marine method, disclosed in U.S. Pat. No. 4,641,287, a series of seismic interrogation pulses are fired by an energy source. The distance to a sensor is determined for each shot by defining a spherical surface upon which the sensor must be located. The intersection of the spherical surfaces derived from a plurality of shots determines the exact location of the sensor. Depth detectors may be used to eliminate one half of the possible locations for each shot.
The above two references are typical of known methods for locating the positions of sensors with respect to an energy source, but the application is limited to marine operations. Specialized equipment is needed such as the sonar system of the '528 patent or the special pinger boat of the '287 patent. Furthermore, neither system would be suitable for land use.
A fatal problem with the known systems is the fact that is assumed by the user that an interrogation signal as received at a sensor is a clean Dirac function. That is, it is assumed that the received acoustic signal is recorded as a sharp spike whose arrival time can be picked with precision relative to the instant of emission of the interrogation signal. In the real world, that assumption is unrealistic because ambient noise, as well as the filtering effect of the medium through which the sonic interrogation pulse travels, severely degrades and contaminates the received signal. The recorded signal is usually fuzzy and indeterminate.
The function of this disclosure is two-fold: To teach a statistical method for estimating the position of a sensor from the intersection of range loci determined from noisy wavefields emitted by a seismic energy source and to provide a method that is equally adaptable to land or marine use without need for special field equipment.